Method and apparatus for inspecting surfaces

ABSTRACT

Method and apparatus for inspecting the surface of a workpiece for the presence of burrs. A focused laser beam is swept across the surface of the workpiece while a photodetector positioned at an acute angle to the workpiece surface senses reflections produced by the beam. The photodetector, when positioned in the plane swept by the beam, observes reflections from the tip of the burr and from the portion of the path on the surface which is not obscured by the burr. The length of the path portion from which reflections are obscured by the burr is directly proportional to the height of the burr. Suitable logic and measuring networks calibrated to the sweep velocity on the workpiece and triggered by the reflections are capable of determining the length of the obscured portion and therefore the height of the burr.

United States Patent Bardos [4 1 Oct. 3, 1972 METHOD AND APPARATUS FORINSPECTING SURFACES [72] Inventor: Andras M. Bardos, 28 Stafford Avenue,Bristol, Conn. 06010 221 Filed: Aug. 6, i970 21 Appl.No.: 61,580

Primary ExaminerRonald L. Wibert Assistant Examiner0rville B. Chew, IIAttorneyFishman and Van Kirk ABSTRACT Method and apparatus forinspecting the surface of a workpiece for the presence of burrs. Afocused laser beam is swept across the surface of the workpiece while aphotodetector positioned at an acute angle to the workpiece surfacesenses reflections produced by the beam. The photodetector, whenpositioned in the plane swept by the beam, observes reflections from thetip of the burr and from the portion of the path on the surface which isnot obscured by the burr. The length of the path portion from whichreflections are obscured by the burr is directly proportional to theheight of the burr. Suitable logic and measuring networks calibrated tothe sweep velocity on the workpiece and triggered by the reflections arecapable of determining the length of the obscured portion and thereforethe height of the burr.

10 Claims, 3 Drawing Figures PATENTED BN3 I972 3.695, 771

FIG. LASER M INVENTOR AN DRAS M. BARDOS ATTORNEYS METHOD AND APPARATUSFOR INSPECTING SURFACES BACKGROUND OF THE INVENTION 1. Field of theInvention This invention relates to the field of inspection equipmentand is more particularly directed to equipment employed for inspectingmachined workpieces for the presence of burrs.

2. Description of the Prior Art In the process of metal working, aworkpiece may be subjected to multiple cutting operations to formvarious contours on the exterior of the workpiece. Various cutting toolssuch as hobbing machines, millingcutters and lathes may be employed togenerate the desired configurations. At the intersection of the surfacesformed by the cutting tools, it is very possible that burrs will beproduced as a result of the cutting action of various tools. Burrs maybe found in the middle of a machined surface as well. Burrs areundesirable since they may interfer with close tolerances generated bythe cutting tools and also because they are hazardous to personnel.Burrs are jagged and sharp and may cause injury to the person orclothing which comes in contact with the burr.

It is particularly important, where close tolerance work is involved, toknow precisely whether a burr is present and possibly even the size ofthe burr. The presence of any burr whatsoever maybe detrimental to theintended use of certain workpieces and may require special grindingoperations if the burr must be moved. The size of the burr is importantin instances where tolerances are critical.

Accordingly, it is desirable to be able to inspect a workpiece for thepresence of a burr and to obtain some knowledge concerning the size oftheburr. Unfortunately, burrs are generally quite small and difficult tomeasure due to their irregularity. Furthermore, if the surface of theworkpiece to be inspected is extensive, the task of examining thesurface for burrs may be time consuming and expensive. It is accordinglydesirable to have an instrument which can automatically examine aworkpiece surface for the presence of burrs and determine the height ofsuch burrs where their. presence is indicated.

It is, therefore, an object of the present invention to disclose amethod and apparatus by which a burr on a workpiece can be detected.

It is a further object of the present invention to disclose apparatuswhich is capable of determining the height of a burr on a workpiece.

It is still a further object of the present invention to discloseapparatus which employs a focused laser beam to provide an accuratemeasurement of the height of a burr.

SUMMARY OF THE INVENTION The novel apparatus and method by which a burrcan be detected and measured employs a focused laser beam which isscanned across the surface of a workpiece at points where burrs might beexpected. A photodetector is positioned at one side of the workpiece andbehind the laser beam as it is scanned at a known rate. Thephotodetector is positioned with a sensitive axis directed toward thesurface of the workpiece being examined in the plane swept by the beamand elevated at an acute angle above the workpiece surface. As the laserbeam is scanned across a burr and on the adjacent workpiece surface, thephotodetector senses reflections of the laser beam as the beam traversesthe tip of the burr and reflections from the workpiece surface which arenot obscured by the burr in the field of .view of the photodetector. Forexample, the region immediately behind the burr with respect to thedetector is hidden from the detector and no reflections will be sensedby the detector until the scanning beam emerges from the obscuredregion. As the beam emerges, the detector again receives reflectionsfrom the path on the surface scanned by the beam. Of course, as isapparent from the drawings, it is the diffuse reflection from thesurface of the workpiece which is sensed by the detector, and referencesherein to reflections from the workpiece surface I will be understoodtobe suchdiffuse reflections. If the velocity at which the beam scans theworkpiece is known, the

length of the scanning pathobscured by the burr can be determined froman examination of the time interval between reflections sensed by-thedetector at the tip of the burr and the exit of the beam fromtheobscured region. Since the length of the scanning path obscuredisdirectly proportional to the height of the burr. the

height ofthe burr can be determined from the time in terval.

The measuring and logic network employed for determining theobscuredpathportion includesa level detector having a selected thresholdwhich starts a timer. The threshold of the level detector is selected sothat only the flash obtained from'the tip of the burr initiates timing.A reduced thresholdin the circuitry is subsequently established so thatless intense reflections from the flat surface of the workpiece can bedetected as the beam emerges from the obscured region.

BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF. THE PREFERREDEMBODIMENTS FIG. 1 is aelementary representation of the burr de tectionapparatus showing the relationship of the scanning laser beam L, theworkpiece W which is to be examined for the presence of a burr B, thephotodetector 10 which senses light from the laser beam L reflected offof the burr and the flat surfaces of the workpiece W and the measuringand logic circuits 11 which control the beam scan and determine burrheight.

The detector 10 has a photosensitive element which senses light radiatedalong the sensitive axis 12 positioned at an acute angle a with respectto the upper surface of the workpiece W. From the positionalrelationship of the detector 10 and the workpiece W, it will be notedthat the detector 10 cannot receive reflections from the surface of theworkpiece within the burr shadow indicated by the shaded region inFIG. 1. It should be understood that the term shadow refers to thatregion behind the burr with respect to the detector 10 which is obscuredfrom the field of view of detector 10 due to the burr B.

From the geometry in FIG. 1, it will be evident that the length of thescanning path which falls within the shadow will be equal to vAt, wherev represents the scanning rate of the beam across the surface of theworkpiece and At represents the time required for the beam to scan fromthe tip of burr B to the end of the obscured region. It is assumed thatthe beam scans at a constant rate to the right in FIG. I as would be thecase where scanning was produced by a saw tooth generator. It will benoted that the burr height, h is directly proportional to the length ofthe obscured portion of the scanning path. The relationship between burrheight and the obscured portion is represented by equation 1 as follows:

From the above equation I it will be immediately recognized that byscanning the beam L at a known velocity or rate established by a sawtooth generator, the height of the burr can be determined simply bymeasuring the time interval during which the photodetector observes noreflections from the workpiece W. The photodetector I is accordinglyemployed for this purpose.

Reference to FIG. 2 reveals the signals produced by the photodetector I0as the laser beam L is scanned across the burr B at the edge of theworkpiece and then onto the surface of the workpiece W. As the beam Lsweeps over the tip of the burr at time t,, represented in FIGS. I and2, the detector receives a flash of reflected light which produces thepulse P. The leading edge of the pulse P is substantially vertical dueto The instantaneous flashof the light reflected off the tip of the burrB. As the beam passes over the burr, the reflected energy decays towardthe null indicating that the beam has passed into the shadow at therighthand side of the burr B in FIG. 1. As the beam continues throughthe shadow, no signal is received or sensed by the detector 10 since theburr obscures any illumination reflected from the impingement point ofthe beam within the shadow." As the laser beam L emerges from the shadowat time t the detector 10 again receives reflected illumination from thescanning path of the beam on the surface of workpiece W.

It will be noted that the level of the illumination provided from thegenerally flat surface of the workpiece is substantially less than thepeak value of pulse P. This is due to the fact that the tip of the burrB produces a substantially stronger reflection of the laser beam L thanthe illumination reflected from the flat surface of workpiece W. If thelaser beam L is scanned across a burr and onto the surface of theworkpiece to-be examined, the initial flash of reflected light from theburr may be used to trigger a timer as thebeam traverses the tip of theburr. The emergence of the laser beam from the obscured portion of thescanning path can also be detected by photodetector 10 to stop the timertriggered by the pulse P. As indicated in FIGS. 1 and 2, the timer wouldbe started at time t and would be stopped at time It follows that Asseen from equation 1 At is directly proportional to the burr height h,,and therefore the measured time interval may be used as a directmeasurement of burr height h The measuring and logic circuits by whichthe scanning and timing functions can be performed are revealed indetail in FIG. 3.

The laser beam L is produced by the beam generator 14 which may bea'laser which produces a focused beam of coherent light. A saw toothsweep generator 16 is shown schematically connected to the beamgenerator 14 to cyclically scan the beam at a known rate from left toright as seen in FIG. 3. The beam would therefore cross over the edgecontaining the burr and move onto the upper surface of the workpiece W.

It will be noted that the photodetector 10 is positioned generallybehind the beam L as the beam sweeps from left to right so that the timerelationship of the pulses received by the detector is as shown in FIG.2. The sensitive axis 12 of the photodetector 10 will lie generally in aplane which is swept by the beam L.

As noted above with respect to FIG. 2, the initial pulse of lightreflected off the tip of the burr B is substantially larger than thegenerally steady reflected light level received from the surface of theworkpiece W due to the beam L or other light sources. Since the peakvalue of the pulse is generally higher than reflections from the flatsurface of the workpiece W, it is possible to establish a thresholdlevel in the measuring circuitry which exceeds the general level ofreflected light at the workpiece surface and therefore preventstriggering of a timer unless a burr does exist at the edge of theworkpiece W. In other words, unless the initial reflection from theworkpiece W is a large pulse produced by a burr, the timer will not betriggered. For this purpose a level detector 18 having an elevatedsignal sensing level, indicated by the line s in FIG. 2, is used as agating mechanism for timer starting pulses in conjunction with abistable flip-flop 20 and AND gate 22. The output of level detector 18is connected to the set input of flip-flop 20 to turn the flip-flop onand the output of the flip-flop is connected to AND gate 22. Thesimultaneous appearance at AND gate 22 of a pulse directly from detector10 and also from flip-flop 20 triggers AND gate 22 and produces anoutput pulse which drives shaping amplifier 24 into saturation. It is,of course, assumed that the components are solid state components whichrespond so rapidly that the leading edge of the pulse P in FIG. 2 willcorrespond substantially with the leading edge of the pulse from ANDgate 22 and saturation of amplifier 24. The saturation of amplifier 24excites single shot multivibrator 25 and produces a narrow pulse whichis conducted to the set" input of bistable flip-flop 26. At the sametime and for reasons to be described hereinafter, the pulse is alsoconducted to a two-bit counter 28. The pulse received by flip-flop 26turns the flip-flop on which produces a signal at one of the inputs toAND gate 30. The second input to AND gate 30 is received from a clockpulse generator 32. Pulse generator 32 may be a multivibrator whichproduces a continuous train of clock pulses at a pre-establish ratesubstantially greater than the sweep frequency of generator 16. Withflip-flop 26 turned on, the output of AND gate 30 is a reproduction ofthe clock pulses from generator 32 and these pulses are fed to counter34. Counter 34 may be a standard digital counter connected to aresetable display device 35.

Summarizing briefly, it is apparent that the leading edge of the pulse Pshown in FIG. 2 triggers AND gates 22 and 30 to allow clock pulses fromgenerator 32 to operate counter 34 and display device 35. In addition,the initial pulse turns bistable flip-flop 20 on so that subsequentreflections received by photodetector l0 and above the threshold levelof the detector and AND gate will pass directly to amplifier .24 whetherthe higher threshold of level detector 18 is reached or not. The initialpulse also is received by two-bit counter 28.

As the laser beam L continues to scan over the burr B and into theshadow region, the signal output of detector 10 drops back to the nullcondition as indicated in FIG. 2. This assumes that the ambient lightlevel is so low that the detector is not responsive. At time the laserbeam L emerges from the shadow in FIG. land produces the detector signalshown in FIG. 2. The level of the signal at the time t is the generalsignal level of reflections from the flat surface of workpiece W due tothe beam and is also sufficient to saturate amplifier 24 as the signalis transmitted directly through AND gate 22 from detector 10. Thesaturated output of amplifier 24 fires single shot multivibrator 25which again pulses two-bit counter 28. The second stage output ofcounter 28 is turned on and is connected to the reset input of bistableflip-flop 26. Flip-flop 26 is shut off by the second stage count andgates circuit 30 off to terminate pulsing of counter 34. Since theresponse of the components is substantially simultaneous, counter 34 isshut off at time The count on the display device 35 at this point istherefore the difference between time t, and time equal to At.

Since the display on device 35 is equal to At the display is alsoproportional to burr height, h,,, as indicated by equation 1. Of course,the count itself may be used as a direct reading of the burr height inan arbitrary set of units.

Since it is desirable to examine the workpiece along an entire edge, theworkpiece can be mounted on a table moving transversely of the beamsweep path while the sweep generator 16 continuously sweeps the laserbeam L back and forth across the edge of the workpiece underexamination. With such a continuous operation, it is necessary to resetthe components in the measuring and logic circuit 11 at the end of eachsweep cycle. For this purpose, differentiation circuit 36 receives thesaw tooth sweep signal and triggers the resetting pulse generator 38 atthe termination of each sweep cycle. Pulse generator 38 may be a singleshot multivibrator which, in response to the leading edge of the pulsefrom differentiation circuit 36, generates a single output pulse havinga relative short pulse width. The output pulse from generator 38 isapplied to the reset input terminals of bistable flip-flop circuits 20and .26. It will be noted that flip-flop 26 may already have been shutoff by the second stage output of counter 28 at time however, the pulsefrom generator 38 is also supplied to the reset input'to insure that theflip-flop 26 has been reset for a subsequent sweep of the beam L.

The pulse from generator 38 is additionally conducted to the two-bitcounter 28 and display device 35. In each case, the pulse resets thecounts to zero for a subsequent sweep of the beam. With the flip-flops20 and 26 of the counters .28 and 34 reset, the system is returned toits initial condition in preparation for a subsequent sweep of the beam.

It will be realized that from the above circuitry, the beam willcontinuously sweep back and forth across the edge of the workpiece W andproduce a count representive of the height of the burr B at points alongthe edge. The sweep rate of generator 16 is low compared to the pulserate of generator 32 and permits the count on display 35 to be observedor recorded. It may be desirable to record the count on a strip chartrecorder and at the same time record information indicative of thelocation of the burr along the workpiece under examination. It will alsobe understood that the burrs being detected may be displaced inwardlyfrom a sharp, burr-free edge of the workpiece and the system willoperate in substantially the same manner as described above.

While a preferred embodiment of the burr detecting equipment has beendescribed and shown in the drawings, it will be understood that variousmodifications and substitutions can be made to the specific componentswithout departing from the spirit of the invention. For example, it willbe readily apparent to those skilled in the an that the beam may becaused to sweep from right to left as seen in FIGS. 1 and 3 and the timeAt would not be varied. Of course, appropriate alterations of themeasuring and logic circuit would be made to trigger and cut off thetimer or counter 34 in response to the signals produced by the output ofdetector 10 in reverse order to that presented in FIG. 2. It may befeasible in certain instances to hold the beam stationary and translatethe workpiece cyclically under the beam. Although a laser beam is shownand described as the scanning beam, other light beams such as that froma zirconium arc lamp may be used with similar results. In general, thelaser beam is preferred because a laser produces a bright, small spotapproaching a point source which is desired for accuracy. The presentinvention, therefore, has been described in a preferred embodiment byway of illustration rather than limitation.

What is claimed is:

1. Apparatus for detecting burrs on a surface comprising:

means for generating an illuminating beam;

means connected to the generating means for scanning the beam at a knownrate along a path across the surface to be inspected;

illumination sensing means for detecting diffuse illumination reflectedfrom the path on the surface swept by the illuminating beam, saidillumination sensing means having a directionally sensitive axis in aplane scanned by said illuminating beam and at an acute angle to thesurface scanned by said illuminating beam whereby light from saidilluminating beam may be obscured from said sensing means in a region onsaid surface behind a burr on the side of the burr removed from saidsensing means during part of the scanning of said beam across saidsurface; and

measuring means connected to the illumination sensing means andresponsive to the signals from the sensing means for measuring theinterval between successive reflections detected by said sensing meansfrom the path scanned by the beam.

2. The detecting apparatus of claim 1 wherein:

the measuring means includes a level detecting means connected to theillumination sensing means and having a first preselected thresholdlevel rendering the measuring means selectively responsive to reflectedillumination.

3. The detecting apparatus of claim 2 wherein:

the measuring means has a second preselected threshold level less thanthe first threshold level of the level detecting means; and

the measuring means includes switching means connected with the leveldetecting means for rendering the measuring means operative at eitherthe first or the second threshold level.

4. The detecting apparatus of claim 3 wherein:

the switching imeans is connected to the output of the level detectingmeans and responsive to the output of the level detecting means forrendering the measuring means operative at the second threshold level asa function of the output of the level detecting means.

5. The detecting apparatus of claim 1 wherein:

the beam generating means is a laser beam generator.

6. The detecting apparatus of claim 1 wherein:

the scanning means scans the beam at a known rate from the edge of thesurface to the path along the surface; the illumination sensing means isa photodetector positioned behind the beam with respect to the directionof scanning and with the directionally sensitive axis directed towardthe scanning beam. 7. The method of inspecting a surface for burrscomprising the steps of:

generating an illuminating beam; sweeping the beam across a surface tobe inspected at a known rate along a path on the surface; detecting, ina plane swept by said beam and at an acute angle with respect to thesurface, illumination diffusely reflected from the path on the surfaceswept by the beam whereby light from said illuminating beam may beobscured from detection at said acute angle in a region on said surfacebehind a burr during part of the sweeping of the beam across thesurface; and measuring the interval between successive detectedreflections from the path. 8. The method of inspecting according toclaim 7 wherein:

the step of measuring includes measuring the time interval between areflection from a burr on the surface and a subsequent reflection fromthe path 9. The method of inspecting according to claim 7 wherein:

the step of generating includes generating a focused laser beam. 10. Themethod of inspecting according to claim 7 wherein:

the step of measuring comprises measuring the time interval betweensuccessive detected reflections above preselected threshold levels, thethreshold level for the first of the reflections being greater than thethreshold level for the succeeding reflection.

1. Apparatus for detecting burrs on a surface comprising: means forgenerating an illuminating beam; means connected to the generating meansfor scanning the beam at a known rate along a path across the surface tobe inspected; illumination sensing means for detecting diffuseillumination reflected from the path on the surface swept by theilluminating beam, said illumination sensing means having adirectionally sensitive axis in a plane scanned by said illuminatingbeam and at an acute angle to the surface scanned by said illuminatingbeam whereby light from said illuminating beam may be obscured from saidsensing means in a region on said surface behind a burr on the side ofthe burr removed from said sensing means during part of the scanning ofsaid beam across said surface; and measuring means connected to theillumination sensing means and responsive to the signals from thesensing means for measuring the interval between successive reflectionsdetected by said sensing means from the path scanned by the beam.
 2. Thedetecting apparatus of claim 1 wherein: the measuring means includes alevel detecting means connected to the illumination sensing means andhaving a first preselected threshold level rendering the measuring meansselectively responsive to reflected illumination.
 3. The detectingapparatus of claim 2 wherein: the measuring means has a secondpreselected threshold level less than the first threshold level of thelevel detecting means; and the measuring means includes switching meansconnected with the level detecting means for rendering the measuringmeans operative at either the first or the second threshold level. 4.The detecting apparatus of claim 3 wherein: the switching means isconnected to the output of the level detecting means and responsive tothe output of the level detecting means for rendering the measuringmeans operative at the second threshold level as a function of theoutput of the level detecting means.
 5. The detecting apparatus of claim1 wherein: the beam generating means is a laser beam generator.
 6. Thedetecting apparatus of claim 1 wherein: the scanning means scans thebeam at a known rate from the edge of the surface to the path along thesurface; the illumination sensing means is a photodetector positionedbehind the beam with respect to the direction of scanning and with thedirectionally sensitive axis directed toward the scanning beam.
 7. Themethod of inspecting a surface for burrs comprising the steps of:generating an illuminating beam; sweeping the beam across a surface tobe inspected at a known rate along a path on the surface; detecting, ina plane swept by said beam and at an acute angle with respect to thesurface, illumination diffusely reflected from the path on the surfaceswept by the beam whereby light from said illuminating beam may beobscured from detection at said acute angle in a region on said surfacebehind a burr during part of the sweeping of the beam across thesurface; and measuring the interval between successive detectedreflections from the path.
 8. The method of inspecting according toclaim 7 wherein: the step of measuring includes measuring the timeinterval between a reflection from a burr on the surface and asuBsequent reflection from the path.
 9. The method of inspectingaccording to claim 7 wherein: the step of generating includes generatinga focused laser beam.
 10. The method of inspecting according to claim 7wherein: the step of measuring comprises measuring the time intervalbetween successive detected reflections above preselected thresholdlevels, the threshold level for the first of the reflections beinggreater than the threshold level for the succeeding reflection.